1. Field of the Invention
The present invention relates to a system for measuring the wavelength dispersion and nonlinear coefficient of an optical fiber.
2. Related Art
With the liberalization of the field of information communication and the development of information society, there is a tendency for the amount of information to increase exponentially. By practical use of erbium-doped fiber amplifiers (EDFAs) and Raman fiber amplifiers that directly amplify light, optical signals of extremely high power can be obtained in the band of wavelength 1.55 μm. This can make up for the transmission losses in optical fibers and make repeaterless transmission possible over thousands of kilometers. By employing such a light amplification technique, wavelength-division multiplexing (WDM) and time-division multiplexing (TDM) have been investigated. Also, in optical repeatered systems employing optical amplifiers, nonlinear optical effects become problems as optical signals of high power pass through optical fibers. Nonlinear effects that arise within optical fibers are self-phase modulation (SPM), cross-phase modulation (CPM), four-wave mixing (FWM), etc., which adversely affect transmission quality. The magnitude of nonlinear effects is determined by the nonlinear coefficient of an optical fiber, so it is necessary to measure the value with a high degree of accuracy. The nonlinear coefficient is expressed by the following Equation:
  γ  =                    2        ⁢                                  ⁢        π            λ        ⁢                  n        2                    A        eff            where λ is the wavelength of light, n2 is the nonlinear refractive index of an optical fiber, and Aeff is the effective area of the optical fiber. The nonlinear effects in optical fibers are evaluated by the value of γ or n2/Aeff.
In a method of measuring the nonlinear coefficient, it can be calculated by causing the pulsed light from a light source to enter an optical fiber, and measuring phase modulation from a change in a power spectral waveform caused by self-phase modulation (see R. H. Stolen and Chinlon Lin, Physical Review A, Vol. 17, No. 4, pp. 1448-1453 (1992)). This method generally is called self-phase modulation method (hereinafter referred to as SPM method).
In addition, by employing two light sources that generate two optical signals with different wavelengths, coupling the two optical signals into a beat pulse, and causing the beat pulse to enter an optical fiber to be measured, the nonlinear coefficient of the optical fiber can be calculated from a change in the light spectrum caused by SPM. This method generally is called CW-SPM method.
Furthermore, by causing probe light and frequency-modulated pump light to enter an optical fiber, detecting the probe light through delayed self-heterodyne detection, and detecting the phase of the probe light modulated by the pump light, the nonlinear coefficient of the optical fiber can be calculated (see A. Wada et al., ECOC92, p. 42 (1992)). This method generally is called cross-phase modulation method (hereinafter referred to as XPM method).
In WDM communication, the most important nonlinear effect is four-wave mixing (FWM). If side bands caused by FWM overlap the wavelengths of other signals, they will generate crosstalk among optical channels, and noise that can limits WDM systems. FWM tends to occur in optical fibers whose nonlinear effects are great, and it is also known that particularly, when an optical signal is in the vicinity of the zero-dispersion wavelength of an optical fiber, FWM can take place easily.
On the other hand, the suppression of FWM against wavelength dispersion in optical transmission lines conflicts with the conditions of waveform distortion due to wavelength dispersion. To solve such a problem, there has been proposed a dispersion compensating system in which local wavelength dispersion in an optical transmission line is not zero, but an optical fiber with positive wavelength dispersion and another optical fiber with negative wavelength dispersion are combined so that the wavelength dispersion of the entire transmission line becomes close to zero.
In designing and constructing such a dispersion compensating system, it is necessary to design the nonlinear and dispersion properties of an optical transmission line with a high degree of accuracy. In design, an optimum fiber combination is often obtained from the average wavelength dispersion and fiber length of an optical fiber.
However, the wavelength dispersion of an optical fiber being actually used is not always uniform at a certain wavelength in the fiber longitudinal direction. The prime cause is errors in manufacturing optical fibers and cables, etc. For that reason, in high data rate and high spectral density transmission with strict restrictions, there are cases where a logical design does not coincide with the state of transmission in optical fibers being practically used.
Therefore, in such a case, the wavelength-dispersion distribution in the longitudinal direction of an optical fiber employed in a dispersion compensating system needs to be grasped accurately and reflected in design.
Methods of measuring the wavelength-dispersion distribution in the longitudinal direction of an optical fiber have been studied in view of the circumstances mentioned above. So far, an optical time-domain reflectometer (OTDR) utilizing linear effects, a method utilizing FWM, etc., have been proposed. A description will hereinafter be given of the principles of the method, utilizing FWM, which measures wavelength-dispersion distribution. The most widely used measurement method measures the back-scattered light of idler light generated by FWM within an optical fiber, and calculates dispersion from the cycle of the power variation (see Optics Letters 1996, 21, pp. 1724-1726 and Japanese Laid-Open Patent Publication No. Hei 8-21783). This method is called a nonlinear OTDR.
Normally, equations describing degenerated four-wave mixing (DFWM) are nonlinear coupled-mode Eqs. (1) to (3).
                                                        ⅆ                              E                p                                                    ⅆ              z                                +                                    1              2                        ⁢            α            ⁢                                                  ⁢                          E              p                                      =                  i          ⁢                                          ⁢          γ          ⁢                                                                                  (                                                                                                                                      E                          p                                                                                            2                                        +                                          2                      ⁢                                                                                                                              E                            s                                                                                                    2                                                              +                                          2                      ⁢                                                                                                                              E                            c                                                                                                    2                                                                              )                                ⁢                                  E                  p                                            +                              2                ⁢                                  E                  p                  *                                            +                              2                ⁢                                  E                  p                  *                                ⁢                                  E                  s                                ⁢                                  E                  c                                ⁢                                  exp                  ⁡                                      (                                          ⅈ                      ⁢                                                                                          ⁢                      Δ                      ⁢                                                                                          ⁢                      β                      ⁢                                                                                          ⁢                      z                                        )                                                                                                                      (        1        )                                                                    ⅆ                              E                s                                                    ⅆ              z                                +                                    1              2                        ⁢            α            ⁢                                                  ⁢                          E              s                                      =                  i          ⁢                                          ⁢          γ          ⁢                                                                                  (                                                                                                                                      E                          s                                                                                            2                                        +                                          2                      ⁢                                                                                                                              E                            c                                                                                                    2                                                              +                                          2                      ⁢                                                                                                                              E                            p                                                                                                    2                                                                              )                                ⁢                                  E                  s                                            +                                                E                  c                  *                                ⁢                                  E                  p                  2                                ⁢                                  exp                  ⁡                                      (                                                                  -                        ⅈ                                            ⁢                                                                                          ⁢                      Δ                      ⁢                                                                                          ⁢                      β                      ⁢                                                                                          ⁢                      z                                        )                                                                                                                      (        2        )                                                                    ⅆ                              E                c                                                    ⅆ              z                                +                                    1              2                        ⁢            α            ⁢                                                  ⁢                          E              c                                      =                  i          ⁢                                          ⁢          γ          ⁢                                                                                  (                                                                                                                                      E                          c                                                                                            2                                        +                                          2                      ⁢                                                                                                                              E                            p                                                                                                    2                                                              +                                          2                      ⁢                                                                                                                              E                            s                                                                                                    2                                                                              )                                ⁢                                  E                  c                                            +                                                E                  s                  *                                ⁢                                  E                  p                  2                                ⁢                                  exp                  ⁡                                      (                                                                  -                        ⅈ                                            ⁢                                                                                          ⁢                      Δ                      ⁢                                                                                          ⁢                      β                      ⁢                                                                                          ⁢                      z                                        )                                                                                                                      (        3        )            where p is pump light, s is probe light, c is idler light, α is the degree of loss in an optical fiber, and γ is the nonlinear coefficient of the optical fiber (γ=(2π/λ) (n2/Aeff) where λ is the wavelength of light, n2 is the nonlinear refractive index of the optical fiber, and Aeff is the effective cross section of the core of the optical fiber). Δβ is the phase mismatching condition of a propagation constantΔβ=βs+βc−2βp  (4)With regard to frequency, Δβ is assumed to meet the phase matching condition2ωp=ωs+ωc  (5)where ωp, ωs, and ωc are the angular frequencies of pump light, probe light, and idler light, respectively.
Because it is difficult to calculate the strict solutions of Eqs. (1) to (3) (nonlinear coupled-mode equations), consideration is given by employing the following approximate solutions.
1) Transmission losses do affect as FWM occurs.
2) The effects of SPM and XPM are not considered.
In view of these, the following Equations (6) to (8) are obtained.
                                          ⅆ                          E              p                                            ⅆ            z                          =        0                            (        6        )                                                      ⅆ                          E              s                                            ⅆ            z                          =        0                            (        7        )                                                      ⅆ                          E              c                                            ⅆ            z                          =                  i          ⁢                                          ⁢          γ          ⁢                                          ⁢                      E            s            *                    ⁢                      E            p            2                    ⁢                      exp            ⁡                          (                                                -                                                                          ⁢                  ⅈ                                ⁢                                                                  ⁢                Δ                ⁢                                                                  ⁢                β                ⁢                                                                  ⁢                z                            )                                                          (        8        )            where Ep is the electric field of pump light of wavelength λp, Es is the electric field of probe light of wavelength λs, Ec is the electric field of idler light of wavelength λc, Pp0 is the power of input pump light of wavelength λp, Ps0 is the power of input probe light of wavelength λs, and Δλ is the spacing between the two input wavelengths.
Therefore, the electric field and power of idler light at a position of z in the longitudinal direction are given by the following Eqs. (9) and (10):
                              E          c                =                                            γ              ⁢                                                          ⁢                              E                s                *                            ⁢                              E                p                2                                                    Δ              ⁢                                                          ⁢              β                                ⁢                      {                          1              -                              exp                ⁡                                  (                                                            -                      ⅈ                                        ⁢                                                                                  ⁢                    Δ                    ⁢                                                                                  ⁢                    β                    ⁢                                                                                  ⁢                    z                                    )                                                      }                                              (        9        )                                          P          c                =                  4          ⁢                      (                                                            γ                  2                                ⁢                                  P                  s                                ⁢                                  P                  p                  2                                                            Δ                ⁢                                                                  ⁢                                  β                  2                                                      )                    ⁢                      sin            ⁡                          (                                                Δ                  ⁢                                                                          ⁢                  β                  ⁢                                                                          ⁢                  z                                2                            )                                                          (        10        )            
In view of transmission losses of pump light, probe light, and idler light, the power of the back-scattered light of idler light received after the fiber distance z is expressed by the following Eq. (11):
                                          P            c                    ⁡                      (            z            )                          ∝                                            (                                                λ                  p                                                  D                  ⁢                                                                          ⁢                  c                  ⁢                                                                          ⁢                  Δ                  ⁢                                                                          ⁢                                      λ                    2                                                              )                        2                    ⁢                                    (                                                                    n                    2                                    ⁢                                      P                    p                    0                                                                    A                  eff                                            )                        2                    ⁢                      RP            s            0                    ⁢                                    sin              2                        ⁡                          (                              Δ                ⁢                                                                  ⁢                β                ⁢                                                                  ⁢                                  z                  /                  2                                            )                                ⁢                      exp            ⁡                          (                                                -                  4                                ⁢                                                                  ⁢                α                ⁢                                                                  ⁢                z                            )                                                          (        11        )            where R=Rayleigh scattering coefficient, α=loss coefficient, and D=dispersion.
The relationship between the phase mismatching condition Δβ and the dispersion D at the wavelength of pump light is expressed by the following Eq. (12):
                              Δ          ⁢                                          ⁢          β                =                                            -                                                λ                  p                  2                                                  2                  ⁢                                                                          ⁢                  π                  ⁢                                                                          ⁢                  c                                                      ⁢                          D              ⁡                              (                                  λ                  p                                )                                      ⁢                                          (                                                      ω                    c                                    -                                      ω                    p                                                  )                            2                                =                                                    λ                p                2                                            2                ⁢                                                                  ⁢                π                ⁢                                                                  ⁢                c                                      ⁢                          D              ⁡                              (                                  λ                  p                                )                                      ⁢                                          (                                                      ω                    p                                    -                                      ω                    s                                                  )                            2                                                          (        12        )            Employing λ=2πc/ω, Eq. (12) can be expressed by the following Eq. (13):
                              Δ          ⁢                                          ⁢          β                =                              -            2                    ⁢                                          ⁢          π          ⁢                                          ⁢                      cD            ⁡                          (                              λ                p                            )                                ⁢                                    (                                                Δ                  ⁢                                                                          ⁢                  λ                                                  λ                  p                                            )                        2                                              (        13        )            And the dispersion D is expressed by the following Eq. (14):
                              D          ⁡                      (                                          λ                p                            ,              z                        )                          =                                            2              ⁢              n                                      c              2                                ⁢                                    (                                                λ                  p                                                  Δ                  ⁢                                                                          ⁢                  λ                                            )                        2                    ⁢                      f            ⁡                          [                              t                =                                                      (                                                                  2                        ⁢                        n                                            c                                        )                                    ⁢                  z                                            ]                                                          (        14        )            where f(t) is the instantaneous frequency of the waveform of the back-scattered light of idler light at time t.
With the high-speed operations and increase in WDM capacity in recent years, the design of optical transmission line requires more strict control. For that reason, the optical transmission line design based on the average wavelength dispersion and nonlinear coefficient of a certain length of fiber, being currently used, needs to consider variations in the wavelength dispersion in the longitudinal direction of an optical fiber. At the same time, when the nonlinear coefficient of an optical fiber also varies in the fiber longitudinal direction, variations in the nonlinear coefficient also affect transmission pulses. For that reason, when making a design more accurately, it is necessary to measure nonlinear-coefficient distribution as well as wavelength-dispersion distribution.
However, at present, there is no means of measuring variations in the nonlinear coefficient in the longitudinal direction of an optical fiber. Therefore, there is no possibility that actual measurement will be made. For that reason, it is unknown how the nonlinear coefficient of an optical fiber being presently used varies and also unknown how the variation affects optical transmission characteristics. Therefore, there is no investigation of how variations in the nonlinear coefficient in the longitudinal direction of an optical fiber affect the design of optical transmission lines and other optical transmission analyses.
However, the nonlinear coefficient of an optical fiber is a significant parameter that characterizes optical transmission characteristics, so it is vital to investigate variations in the nonlinear coefficient in the longitudinal direction of an optical fiber. Because of this, there is a strong demand for the development of a method and system for measuring the properties (dispersion, nonlinear effects, etc.) of optical fibers that can contribute to the development of next-generation transmission lines, and fiber devices that need to adjust for the wavelength dispersion and nonlinear coefficient.
In addition, a change in the diameter of glass when being drawing into fiber form is considered to be the main cause of variations in the wavelength dispersion in the fiber longitudinal direction produced when manufacturing optical fibers. The refractive-index distribution of the base material of an optical fiber to be drawn into fiber form can be measured by a preform analyzer, so the drawing operation is performed based on the result of measurement so that target wavelength dispersion is obtained. However, since there is an error in the accuracy of finishing in the fiber material or a measurement error in the preform analyzer, not a few variations will arise in the fiber longitudinal direction. Likewise, from the viewpoint of manufacturing optical fibers, a fiber manufacturing method that is stable and good in yield rate is requested, and a fiber-property evaluating method therefor is also requested.